1. Field of the Invention
The present invention relates to light-emitting elements including an organic compound at least as a part thereof. The present invention also relates to lighting apparatuses, light-emitting devices, electronic appliances, and displays which include the light-emitting element.
2. Description of the Related Art
Development of a light emitting device including a light-emitting element in which a layer including an organic compound is provided between a pair of electrodes and light emission is obtained by current flowing between the pair of electrodes has been advanced. Such a light-emitting device can reduce thickness and weight of a display device in comparison with a display device which is currently called a “thin display device”. Moreover, such a light-emitting device is of a self-emission type, and therefore, has a high level of visibility and high response speed. Accordingly, such a light-emitting device has been actively developed as a next-generation display device, and has been partly put into practical use at present.
Such a light-emitting element can emit light in various colors depending on a material serving as an emission center which is included in a layer including an organic compound. Moreover, by stacking layers each including an emission center substance which exhibits different emission colors from each other, light emitted from the layers is mixed, and a wide variety of emission colors can be obtained. In particular, the emphasis is put on white light because white light is suitable for the use of a backlight or lighting in addition to a display. The white light can be obtained by mixing red light, green light, and blue light or mixing emission colors which are in a relationship of complementary colors (see Non-patent Document 1 and Non-patent Document 2).
Deterioration of a light-emitting element is one of the main reasons why such a light-emitting device including the above-described light-emitting element with many advantages is limited to a partial practical use. A light-emitting element deteriorates such that the luminance is lowered with accumulation of a driving time even if a constant current is fed to the light-emitting element. In order to widely promote the light-emitting device, it is necessary to obtain a light-emitting element whose degree of deterioration is acceptable in a product use; therefore, a light-emitting element has been studied from many aspects such as a driver circuit, sealing, an element structure, and a material.
In addition, light-emission efficiency of the light-emitting element is also an important factor. By improving the light-emission efficiency, electric power needed to provide light with given brightness of the same color can be reduced. That is, as energy problems are getting more serious today, if the same brightness can be obtained with smaller energy, products with higher added values can be provided. Further, an improvement in light-emission efficiency can reduce the amount of current flowing into a light-emitting element to emit light with given brightness of the same color, which leads to suppression of deterioration. Although comparison between light of different colors are not easy, quantum efficiency gives an indication.    [Non-patent Document 1] Device physics, Material Chemistry, and Device Application of Organic Light Emitting Diodes, editorial supervisor Chihaya ADACH1, published by CMC Publishing CO., LTD, December, 2007, pp. 257-267.    [Non-patent Document 2] Yin-Jui Lu, et al., “Achieving Three-Peak White Organic Light-Emitting Devices Using Wavelength-Selective Mirror Electrodes”, SID07 Digest, 2007, pp. 1110-1113.